Many metal-containing devices and structures must function in corrosive atmospheres which cause them to deteriorate over time. Corrosion may take the form of metal oxides resulting from reaction with oxygen in the air, or may by compounds formed with the effluent of industrial processes, such as hydrogen sulfide.
In the electronics industry, for example, approximately one-third of all warranty repair work is attributable to corrosion. Accordingly, the ability to accurately monitor corrosion and take appropriate measures to deter its spread are of utmost importance to the industry.
The standard method of monitoring corrosion has historically been accomplished using a reactivity monitoring procedure such as the so-called "coupon" method. Under this method, strips of copper are placed in the environment where corrosion is to be monitored. The coupons carry an initial copper oxide corrosion thickness of about 100 Angstroms (.ANG.). After a period of time in the environment, usually around thirty days, the change in thickness of corrosive buildup on the strips, or coupons, is measured using a complex coulometric reduction procedure, well known to those skilled in the art.
Using an accepted standard such as Standard No. ISA-S71.04-1985 set by the Instrument Society of America (ISA) of Research Triangle Park, N.C., this change in thickness is then projected over a chosen period of time. Other organizations, such as Battelle of Columbus, Ohio, have also developed such standards which, like the ISA standard, are based on reactivity monitoring techniques. Given a corrosive buildup after any number of days, the standard may be applied to project the weekly, monthly, or annual buildup of corrosion in the environment. Such information is vital to the electronics industry in determining the reliability and projected lifetime of equipment. It may affect the scope or duration of warranty coverage, particularly in limiting such coverage when the equipment will be used in corrosive environments. The reactivity monitoring method of corrosion monitoring using coupons is discussed in further detail in "Environmental Conditions and Process Measurement and Control Systems: Airborne Contaminants," a 1985 ISA publication; and Krumbein, Newell, and Pascucci, "Monitoring Environmental Tests by Coulometric Reduction of Metallic Control Samples," Journal of Testing and Evaluation, Vol. 17, No. 6, Nov. 1989 , pp. 357-67, both of which are incorporated herein by reference. Althrough copper, silver, and nickel are part of the electronic circuitry, copper is the only metal addressed by the ISA standard. Accordingly, there is a need in the art to monitor the corrosion of electronics circuitry containing other corrodible metals, in addition to copper. References to corrodible metals herein include any corrodible metal, and also include such metals coated with gold. Examples of corrodible metals, without limitation thereto, are copper, silver, nickel, and laminates of such metals which may or may not be coated with gold.
One major disadvantage of the coupon method of reactivity monitoring, however, is the destructive nature of the measurement. Once the thickness of corrosion on the coupon has been measured, the coupon must be discarded and, although the measurement may be projected over a desired period of time, further actual corrosion measurements may only be taken with a new coupon. Accordingly, there is a need in the art to provide a non-destructive method for measuring corrosion in terms of recognized reactivity monitoring standards.
One possible solution to this problem is to measure the corrosion buildup in terms of frequency change. Such a solution has been disclosed in the prior art writings of Lu and Czanderna, APPLICATIONS OF PIEZOELECTRIC QUARTZ CRYSTAL MICROBALANCES (Elsevier, 1984), pages 203-05; and Lee, Siegmann, and Eldridge, "A Comparison of the Mass and Resistance Change Techniques for Investigating Thin Film Corrosion Kinetics," 124 Journal of the Electrochemical Society (May 1977, pages 1744-47), which are both incorporated herein by reference. The use of a piezoelectric crystal to analyze corrosion is also generally disclosed in U.S. Pat. No. 4,783,987, to Hager et al., also incorporated herein by reference. These references teach the use of a quartz crystal microbalance (QCM) which is attached to an oscillator, from which the frequency of vibration of the QCM is measured. As the metal layered on the quartz crystal corrodes over time, the frequency of the QCM changes, thus providing an indication of corrosion in terms of frequency change.
Measuring the change in frequency of the QCM enables one to conduct real-time measurements of corrosion. Unlike the reactivity monitoring coupon method, which requires a new, unblemished coupon each time the thickness of corrosion is measured, frequency measurements may be repeated time and time again as the corrosion continues to accumulate without destroying the QCM. However, an indication of frequency change does not allow comparison with specifications stated in terms of an accepted industry standard of corrosion measurement which is stated in terms of thickness.
It should be noted that a reactivity monitoring coupon prepared according to the ISA standard has a surface quite different from that of a coated crystal. The ISA standard requires that the coupon, a metal strip, be sanded or abraded. Thus, the coupon presents a rough surface to the corrosive atmosphere. In contrast, the metal layer on a coated crystal is vacuum deposited to form a shiny, smooth surface. Corrosion forms differently on such different surfaces. Therefore, finding a correlation between corrosion on a coated crystal detected by means of a change in the frequency of vibration of the crystal, and the corrosion that would have occurred under the same conditions on a new, unblemished coupon prepared according to a standardized reactivity monitoring procedure, is not a simple or obvious matter.
U.S. Pat. No. 3,253,219, to Littler, describes the use of a piezoelectric crystal to measure the decrease in thickness over time of a test specimen, such as a vinyl acetate resin, which is adhered to the crystal. As the thickness of the specimen decreases, the frequency of vibration of the crystal increases. When a crystal with a 3.5 MHz oscillating frequency is utilized, a decrease in thickness of 1 .ANG. is said to be equivalent to an increase in frequency of 1 Hz. Littler, however, does not address the corrosion of metals, which are the subject of the electronic industry's concerns. Our tests have shown that Littler's teaching regarding the thickness change of eroding plastics does not hold true for corroding metals laminated on the vibrating crystals we have tested. Thus, there has been a need in the art for a corrosion monitor using a piezoelectric crystal that can report corrosion measurements in terms of an accepted reactivity monitoring standard. Furthermore, Littler does not suggest or disclose any means for monitoring, generating, or displaying the thickness of corrosion. Importantly, Littler does not address the impact of atmospheric factors, such as air temperature or relative humidity, on the ability to accurately monitor the buildup of corrosion.
U.S. Pat. No. 4,869,874, to Falat, which is incorporated herein by reference, describes a device which measures corrosion, taking into account atmospheric conditions such as temperature, pressure, and humidity by comparing existing conditions to present limits. However, in order to achieve accurate, useful results, Falat requires that the monitoring occur over an extended designated period of time, usually on the order of about six months. There is therefore a need in the art to provide a corrosion monitor that takes atmospheric conditions, such as temperature and humidity, into consideration and provides accurate, useful data on an as-needed basis, as frequently as daily.